FIFTH, OK AGASSICERAN BRANCH. 



205 



Vak. quadragonatum, Hyatt. 



Localities. — Lyme Regis, Semur. 



A form in the Museum of Comparative Zoology, represented in Figures 34, 

 35, closely resembles the stout variety of obtusion, but exaggerates the char- 

 acters of that form and of variety B. Though young, it has a more subquad- 

 ragonal whorl than is usual in the adults of obtusion, var. B, a flatter abdomen 

 and broader channels, and quite distinct piloe, with a tendency to the formation 

 of tuberculated geniculae, though actual tubercles are not present. The diameter 

 is 53 mm. The outer whorl is 20 mm. on the sides, and the transverse diameter 

 the same. The sides are about parallel, the abdomen flat, distinct, sub-angular, 

 and geniculse are present. The abdominal part of the pilse do not bend forward, 

 but pass on to the abdomen and interrupt the channel ridges. The ridges are 

 well marked, though not so prominent as in Turneri. 



It is evidently a modified form of obtusum, in which the pilse by a very slight 

 increase in the geniculas would become tuberculated. Dumortier, in his " Etudes 

 Pal. Bassin du Rhone," describes a specimen of obtusum having eight or nine 

 irregular nodes upon the young whorls until 20 mm. in diameter, and gives these 

 as characteristic of the young of his specimens. They probably belonged to this 

 variety, in which the young are more strongly marked in this way than is usual 

 in other varieties of obtusum} 



There is a large specimen in the Museum of Comparative Zoology, from Lyme 

 Regis, 500 mm. in diameter. Though considerably altered by pressure, the last 

 whorl has the aspect of var. quadragonatum, and the normal form is comparatively 

 slightly altered on the third quarter of this volution. It is here only 115 mm. in 

 abdomino-dorsal diameter, which is considerably less than would occur in any 

 equally large specimen of other varieties. There are pseudo tubercles on the 

 outer whorl, but these may in large part be due to distortion of the pila3 by 



1 The similarity of Arietites stellmformis, as figured by W'ahner, Unter. Lias, VI. pi. xxv., to Asteroceras 

 may be simply a case of parallelism, due to clinologic degeneration in some species, which may, however, 

 belono- to an entirely different genus. The umbilical whorls are heavily though sparsely tuberculated, and 

 the outer whorls are, in our opinion, those of an old shell. See also page 100, note 3, above. 



